Music can be an important part of an exercise or other physical activity routine for many people. For example, runners, weight lifters, participants in aerobic exercises, users of cardiovascular exercise machines (e.g. stair steppers, stationary bicycles, elliptical motion machines, and the like), etc. frequently listen to music while participating in such activities. Music can help alleviate boredom, and a particularly motivational or inspirational song or group of songs can be helpful in maintaining focus and improving performance. However, while a user can assemble a group of songs (also referred to as a playlist or “mix”) using one or more sources such as a personal music collection, an online music streaming service, or the like, such playlists generally lack the ability to react to the specifics of the physical activities that the user is performing at any given time.